1. Field of the Invention
This invention relates to a method of producing a cooperating pair of punch and die used for pressing-forming a blank material into a panel product.
2. Description of the Related Art
various parts of a motor vehicle, such as a fuel tank of a motorcycle, are produced by press working. In the press working, a press-forming die assembly formed from a precision-machined alloy steel is used in general. The alloy steel press-forming die assembly is highly durable and hence is able to recover its manufacturing cost when used in mass manufacturing systems of vehicle parts.
In recent years, the model change cycle of motorcycles has become shorter than as before, and the manufacturing system tends to shift from mass manufacturing of one type of product to more flexible manufacturing of a wide variety of products with relatively low volume of production. The alloy steel press-forming die assembly, when used in flexible manufacturing systems, may sometimes fail to recover its manufacturing cost, leading to a cost-up of the final product.
With the foregoing problem in view, a press-forming die assembly formed from a synthetic resin is generally used for flexible manufacturing systems. The synthetic resin press-forming die assembly is relatively easy to manufacture and can be manufactured at a low cost as compared to the alloy steel press-forming die assembly. Accordingly, even when the model change cycle of the motorcycle is relatively short, the synthetic resin press-forming die assembly is able to recover its manufacturing cost and hence prevents a cost-up of the final product.
However, a working surfaces of the synthetic resin press-forming die assembly is softer than that of the alloy steel press-forming die assembly and hence is susceptible to wear in a relatively short period of use. Once wear occurs on the working surfaces, it is difficult to maintain the desired accuracies of the press-formed products. Accordingly, in order to maintain the desired product accuracies, the synthetic resin press-forming die assembly must be replaced before the working surface is worn away. With is requirement, the synthetic resin press-forming die assembly is replaced in a relatively short cycle and may sometimes fails to recover its manufacturing cost.
Another known relatively low-cost press-forming die assembly is made of a zinc alloy generally called xe2x80x9cZASxe2x80x9d. The ZAS is a zinc-based alloy containing aluminum and antimony and hence has a relatively low melting pint such as about 380xc2x0 C. Because of this low melting point, the ZAS press-forming die assembly can be manufactured at a relatively low cost.
However, a working surface of the ZAS press-forming die assembly is relatively soft and hence is poor in durability as compared to that of the alloy steel press-forming die assembly. Due to a relatively short replacement cycle, the ZAS press-forming die assembly may sometime fail to recover its manufacturing cost.
There has been a keen demand for a press-forming die assembly which can be manufactured at a reduced cost and has high durability comparable to that of the alloy steel press-forming die assembly.
It is accordingly an object of the present invention to provide a method of producing a highly durable press-forming die assembly (punch and die) at a relatively low cost.
According to a first aspect of the present invention, there is provided a method of producing a press-forming die, comprising the steps of: preparing a master die and a master punch relatively movable toward and away from each other so as to define therebetween a die cavity, the master die having a working surface partially defining the die cavity; while a first blank sheet is disposed between the master die and the master punch, relatively reciprocating the master die and the master punch together, thereby press-forming the first blank sheet into a press-formed panel complementary in contour to the shape of the die cavity; cutting down the working surface of the master die by a predetermined depth of cut; fitting the press-formed panel over the master punch; while a second blank sheet having a thickness equal to the predetermined depth of cut of the working surface is disposed between the master die and the press-formed panel fitted over the master punch, relatively reciprocating the master die and the master punch together, thereby press-forming the second blank sheet into a die panel complementary in contour to the shape of the press-formed sheet; and applying a back-up material to the rear surface of the die panel to thereby produce a press-forming die.
According to another aspect of the present invention, there is provided a method of producing a press-forming punch, comprising the steps of: preparing a master die and a master punch relatively movable toward and away from each other so as to define therebetween a die cavity, the master punch having a working surface partially defining the die cavity; while a first blank sheet is disposed between the master die and the master punch, relatively reciprocating the master die and the master punch together, thereby press-forming the first blank sheet into a press-formed panel complementary in contour to the shape of the die cavity; cutting down the working surface of the master punch by a predetermined depth of cut; fitting the press-formed panel in the master die; while a second blank sheet having a thickness equal to the predetermined depth of cut of the working surface is disposed between the master punch and the press-formed panel fitted in the master die, relatively reciprocating the master die and the master punch together, thereby press-forming the second blank sheet into a punch panel complementary in contour to the shape of the press-formed sheet; and applying a back-up material to the rear surface of the punch panel to thereby produce a press-forming punch.
According to a third aspect of the present invention, this provided a method of producing a press-forming punch and a press-forming die in succession, comprising the steps of: preparing a master die and a master punch relatively movable toward and away from each other so as to define therebetween a die cavity, the master die and punch having respective working surfaces jointly defining the die cavity while a first blank sheet is disposed between the master die and the master punch, relatively reciprocating the master die and the master punch together, thereby press-forming the first blank sheet into a press-formed panel complementary in contour to the shape of the die cavity, cutting down the working surface of the master punch by a predetermined depth of cut; fitting the press-formed panel in the master die; while a second blank sheet having a thickness equal to the predetermined depth of cut of the working surface is disposed between the master punch and the press-formed panel fitted in the master die, relatively reciprocating the master die and the master punch together, thereby press-forming the second blank sheet into a punch panel complementary in contour to the shape of the press-formed sheet; repeating the second-named reciprocating step to thereby produce a second punch panel; applying a back-up material to the rear surface of the first punch panel to thereby produce a press-forming punch; cutting down the working surface of the master die by a second predetermined depth of cut; fitting the second punch panel over the master punch; fitting the press-formed panel over the second punch panel; while a third blank sheet having a thickness equal to the second predetermined depth of cut of the working surface of the master die is disposed between the master die and the second punch panel fitted over the second punch panel, relatively reciprocating the master die and the master punch together, thereby press-forming the third blank sheet into a die panel complementary in contour to the shape of the press-formed sheet; and applying a back-up material to the rear surface of the die panel to thereby produce a press forming die.
It is preferable that the method comprises the additional step of applying a backing material to the rear surface of the die panel or the rear surface of the punch panel. The backing material comprises a water-soluble epoxy resin, and preferably a fibre-reinforced water-soluble epoxy resin.
The die panel or the punch panel is preferably made of steel, such as stainless steel or high-tensile steel, and has been subjected to work-hardening processes when press-formed from the corresponding blank sheet. The work-hardened die panel and punch panel have an increased surface hardening and hence they are highly resistant to wear.
The back-up material may be a high-strength cement or synthetic resin. When the high-strength cement is used, the applying step includes placing a slurry of high-strength cement on the rear surface of the die panel or the punch panel while removing bubbles from the slurry by the action of a vacuum. It is preferable that the applying step further includes applying vibrations to the slurry to promote separation of the bubbles from the slurry.
The above and other objects, features and advantages of the present invention will become manifest to those versed in the art upon making reference to the following description and accompanying sheets of drawings in which certain preferred structural embodiments incorporating the principle of the invention are shown by way of illustrative example.